1. Field of the Invention
The invention relates generally to a sensorless control unit for a brushless direct-current (DC) motor, and more specifically to a sensorless control unit for a brushless DC motor that is suitable for driving a pump that sucks in and discharges oil.
2. Discussion of Background
Hydraulic pressure is supplied to a transmission of a vehicle by a hydraulic pump. In a vehicle that carries out so-called idling stop, which is a technique of stopping an engine when the vehicle stops, in view of, for example, energy-saving, an electric hydraulic pump is used to reliably supply hydraulic pressure to the transmission even during idling stop.
A brushless DC motor is used as an electric motor for driving a pump, which is mounted in a vehicle. In addition, so-called sensorless control for driving the motor without the use of a rotational position detection sensor is executed.
In order to execute the sensorless control over the brushless DC motor, it is necessary to estimate the rotational position of a rotor and generate an estimated rotational position signal that corresponds to a rotational position signal from a rotational position detection sensor. The estimated rotational position signal is usually generated with the use of three-phase induced voltages of the motor. However, when the rotor is not rotating or is rotating at a low speed during start-up of the motor, induced voltages are zero or low. Therefore, it is not possible to generate an estimated rotational position signal. Accordingly, the pattern of electric power supply to the three phases is forcibly changed at regular intervals to generate a revolving magnetic field to thereby carry out forced commutation for forcibly rotating the rotor.
A sensorless control unit described in Japanese Patent Application Publication No. 2005-278320 (JP 2005-278320 A) rotates a brushless DC motor at a predetermined rotation speed, determines whether a rotor position is detectable, and makes a switchover from a forced commutation mode to a sensorless control mode when the rotor position becomes detectable.
When the above-described conventional sensorless control unit for a brushless DC motor is applied to a brushless DC motor for driving a hydraulic pump for a transmission of a vehicle, the following problem may occur. That is, if the oil temperature is low (the hydraulic load is high), it is not possible to make a smooth switchover from the forced commutation mode to the sensorless control mode.
That is, the conventional sensorless control unit may be unable to detect a rotor position after making a switchover to the sensorless control upon detection of the rotor position. In this case, it may not be possible to execute the sensorless control. In addition, it is difficult to quickly increase hydraulic pressure during start-up of the motor.
Therefore, in order not to cause a detection error even if the oil temperature is low, the rotation speed of the rotor in the forced commutation mode may be set to be high. In this case, however, there is a problem that the rotor is not able to follow changes in magnetic poles and therefore loss of synchronization is likely to occur.